Download Linking Ground Hydrology to Ecosystems and Carbon Cycle

Present and Future of Modeling Global Environmental Change: Toward Integrated Modeling,
Eds., T. Matsuno and H. Kida, pp. 137–144.
© by TERRAPUB, 2001.
Linking Ground Hydrology to Ecosystems and Carbon Cycle
in a Climate Model
Robert E. D ICKINSON
School of Earth & Atmospheric Sciences, Georgia Institute of Technology,
GA 30332-0340, U.S.A.
The paper discusses land surface processes in a climate model, linking the roles
of hydrology, ecosystems, and carbon cycling. The integrating factor is the
movement of water and carbon dioxide through the stomates of leaves. This
molecular gas transfer is a major control of coupling between land and the
atmosphere, as required for modeling climate variability and change, land storage
of carbon, and the dynamics of ecosystems. Recent work by the author on
parameterizing these fluxes is reviewed. These parameterizations lead to a
framework for calculating leaf areas as a model prognostic variable. It is
necessary to also include the processes responsible for cycling of nitrogen
between leaves and soil for a physically complete description.
Land surface processes are a major element of current comprehensive
climate/earth system models. From the viewpoint of climate variability,
evapotranspiration (ET) is perhaps the most important process that couples land
to atmosphere. Much of the water in ET is transpired through leaves (Dickinson,
1983). Hence, parameterization of the land vegetation addresses the most important
process for moving water from land into the atmosphere. Given precipitation,
evapotranspiration is closely related to runoff and the supplies of water in general
since averaged over time, a change in one requires an equal and opposite change
in the other.
The issue of greenhouse warming requires concentrations of carbon dioxide
in the atmosphere. Because the fluxes of carbon dioxide from fossil fuel burning
are partioned between the atmosphere and land in nearly equal amounts, another
requirement for the parameterization of land processes is to determine the fluxes
of carbon dioxide between land and atmosphere. Land carbon is stored either in
vegetation or in soils; the soil reservoir is almost entirely supplied by dead
vegetation. All such carbon is moved from the atmosphere to land through the
stomates of leaves. It is these same stomates that move water from the soil to the
atmosphere. Indeed, the primary biological reason that plants transpire is that
water necessarily leaks out when carbon dioxide is taken in. Hence, in most
current land parameterizations, water and carbon dioxide fluxes are determined
together from the same leaf model (e.g. Sellers et al., 1997). The leaf assimilation
of carbon and its relationship to ET is the primary input for modeling the growth
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of plants. The relative success of different plant functional types in utilizing their
assimilated carbon determines the composition of both natural ecosystems and
agricultural productivity. Hence, because of these close connections, it is natural
for a climate model to use the same framework for water fluxes, carbon fluxes,
and ecosystem dynamics.
Current climate research addresses both the issues of greenhouse warming
and natural climate variability. There are both strong connections in the dynamics
of the climate system between variability and long term change, and both are
closely related to land water and carbon fluxes. Simple box models of oceanatmosphere energy exchange and land-atmosphere water exchange show that the
longest time scales of variability and the amplitude of variability for given
forcing are determined by the slow leakages of energy and water from dynamical
climate variability systems in which these quantities are otherwise conserved in
exchange between the atmosphere and surface (e.g. Dickinson et al., 2000).
The time scale and response amplitude are given for the ocean-atmosphere
system by the heat capacity of the ocean divided by the net negative radiative
feedback of the atmosphere, and for land by the vegetation determined soil water
capacity divided by the net rate of leakage of water from land to ocean. Sorting
out all the positive and negative atmospheric radiative feedbacks as they depend
on clouds and water vapor among other ingredients is not simple, indeed still the
main limitation to our estimates of temperature change from greenhouse warming.
I suggest that this may likewise be a major obstacle to prediction of climate
variability and that 3-D climate models based on underlying physics that are
successful for projecting global warming should also be good for predicting
climate variability.
Although the simple 2-box models are most easily derived as global averages
they are readily seen to also apply for regionally localized systems and for state
variables averaged over areas with boundaries through which there is no transport.
As used by Saravanan and McWilliams (1998), they are also apply to horizontal
model structures. Ocean-atmosphere systems involving conservation of energy
project onto land and excite the land-atmosphere systems involving conservation
of water or vice versa. Hence, the fluxes of water through land vegetation are
necessarily a major ingredient of modeling climate variability. Such variability
leaves an observed signature in biogeochemical processes, especially carbon
fluxes between land and atmosphere, whose explanation is needed to understand
the role of land as a sink for fossil fuel carbon dioxide.
The linkages of soil hydrology to issues of carbon and ecosystem most
directly involve the role of ET. ET is the primary sink term for soil water since
where precipitation supports soil moisture it also generally supports vegetation.
The soil surface can only evaporate a relatively small amount before surface
dryness limits further upward diffusion of moisture. However, roots penetrate
much deeper into the soil and hence extract soil water from much deeper layers
than could happen with surface evaporation alone. How leaves move water to the
atmosphere is the primary topic discussed here.
Why do plants have to move water from the soil? Only a very small fraction
Linking Ground Hydrology to Ecosystems and Carbon Cycle in a Climate Model
139
of the water used is actually needed to combine with carbon dioxide in the process
of photosynthesis. This requirement for water is primarily dictated by the need of
leaves to be open to the atmosphere to take in carbon dioxide. How much is
determined by the atmospheric demand, the ability of roots to extract water from
the soil, and the mechanisms that control water movement through the plants. On
an average, atmospheric demand is largely dictated by daytime net radiative
heating, but also depends to some extent on the relative humidity of near surface
air. Given that storage terms averaged over time can be neglected, the net
radiative heating must be balanced by evaporative and dry heat energy fluxes
carried upward by boundary layer turbulence. Hence, the ratio of the dry heat flux
to evaporative energy flux, referred to as the Bowen ratio, is a key parameter for
the determination of ET. It also is of major importance for establishing the
convective coupling between land and atmosphere and hence the role of land in
climate variability and change. For wet surfaces and an overlying saturated
atmosphere, this ratio depends only on temperature. The Bowen ratio, under these
conditions, varies from values greater than 1 at relatively cold temperatures
(below 1°C) to values smaller than 1/4 for warm surfaces (above 3°C). A drier
overlying atmosphere increases ET and hence reduces the Bowen ratio from these
values. Conversely, the Bowen ratio is reduced if the underlying surface is drier
than a wet surface, that is, if the air in direct contact with it has a relative humidity
lower than 100%.
The diffusive resistance to water vapor movement from the inside to the
outside of the leaf controls the relative humidity at the surface of leaves. Why do
plants allow any such water loss? They must assimilate carbon dioxide from the
air for photosynthesis and this requirement to take in carbon requires openings in
the leaves called stomates, which are also the primary conduits for water loss.
Hence, modeling the role of stomates in leaf water loss implies also modeling the
leaf assimilation of carbon. Because of this close connection between leaf loss of
water and gain of carbon dioxide, growth of plants is very tightly linked to their
extraction of soil water through roots and leaves. Any gardener knows that for
periods without precipitation plants need to be watered to avoid wilting. However,
even where the plants can protect themselves by preventing transpiration when
the soil is dry, they cannot grow under such conditions as they will not have any
carbon to photosynthesize into sugars, hence, no building blocks for plant tissue
or fuel for their energy requirements. That mechanisms to prevent water loss also
reduce plant growth is evident in the slow growth rates of desert plants that are
designed to conserve water.
The most fundamental factors in the growth of plants are the rate at which
they take up carbon dioxide and the energy exchanges with the atmosphere
determining their temperature. Temperature drives the operation of various
plant-metabolic processes affecting both the rates of carbon assimilation and the
plant requirements for metabolic energy. The supply of soil water to the leaves
along with that of solar energy in turn strongly affects the two fundamental
factors, carbon assimilation and temperature. Hence, soil water and solar energy
provide the basic inputs for modeling of plant growth, and hence the distinctions
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between the different plant types that occur at different latitudes and climates.
Geographers have, in the past, designed static models of Earth’s ecosystems
purely from the good correlations with measurements of air temperature and
precipitation. However, more basic approaches start with the connections between
soil water loss and carbon gain. More detailed models of plant growth must also
address the issue of carbon allocation, that is how much is required to supply plant
energy needs, referred to as plant respiration, and how the remainder is divided
among the different growing tissues. Generally, comparable amounts of carbon
are given to leaves, roots, flowers and seeds, and wood, if a woody plant.
Roots are essential not only for finding the soil water needed by plants to
assimilate carbon but also for extracting from the soil limiting nutrients, especially
nitrogen and phosphate. Hence, some combination of the plant carbon and
nutrient requirements determines how much plant carbon is allocated to the roots
and to what depth the roots mine for carbon and nutrients. Since nutrients are
largely obtained by recycling of previous plant material, limiting nutrients tend
to concentrate near the surface whereas near surface water is the first to be lost
to ET and so roots may have to find water at greater depths. The growth of roots
in turn will move carbon and nutrients downward in the soil column.
Future generations of climate models should be capable of including all
these fundamental Earth system processes. I review here recent efforts of mine to
treat some aspects. Since leaves are fundamental to both water and carbon, they
should be calculated as climate model state variables. Dickinson et al. (1998)
simplified the leaf carbon assimilation parameterization pioneered by Farquhar
et al. (1980), and Collatz et al. (1991).
Stomatal resistances were specified as in earlier treatments of ET to show
that these parameters were largely interchangeable with the more current
specification of maximum photosynthesis rate parameters. The main ingredients
of such a model beside carbon assimilation are the respiration loss terms resulting
from mitochondrial utilization of the carbon to generate energy in the plants and
soil microbiota, the allocation of carbon between leaf tissues and other plant
components, and various factors that convert live plant carbon to carbon stored
in dead plants or soil. Hence, a model for leaf dynamics also provides a carbon
model. However, adequate modeling of carbon fluxes back to the atmosphere
depends on soil biochemical processes that do not directly affect the leaf
dynamics. Currently, the primary data for validating such models of the
geographical and seasonal variation of leaf cover is the NDVI greenness parameter
from the AVHRR instrument imagery from NOAA polar orbiter meteorological
satellites. Since a variety of processes contribute to the observed variations of
greenness, modeling of the satellite data is required to translate it into the leaf area
index parameter (LAI) needed by a climate model and the data may contain
artifacts of the satellite and sun geometries. The primary biophysical roles of
vegetation in a climate model besides the stomatal controls on ET are the
determination of land albedos and surface aerodynamic roughness. In addition,
the amounts of carbon that can be stored in live biomass is needed.
The albedo, roughness, and carbon store depend to a large extent on the
Linking Ground Hydrology to Ecosystems and Carbon Cycle in a Climate Model
141
PAR
Triose
Phosphate
H2O
O2
Enzymes
Phosphoglycerate
wj
Enzymes
we
wc
Rubisco
Ribulose
bis-Phosphate
ATP
ADP
Fig. 1.
height of the vegetation. Both height of vegetation and how it relates to these
needed parameters is not known very well but in principle, can be modeled in
terms of plant functional types and age distributions. For example, for a given
climate whether grass or trees will be the dominant species can be determined
from models of the dynamics of the competition between these systems. The
parameterizations that I have developed have assumed such ecosystem structure
as prescribed boundary conditions, but other authors are addressing how the
dynamics of such systems can be included as part of a climate model (e.g. Foley
et al., 1996).
The biochemistry of leaf carbon assimilation involves a large number of
individual reactions. Farquhar showed that these could be parameterized in terms
of three (3) limiting rates as illustrated in Fig. 1. The first step of the conversion
of CO2 and water to carbohydrate is the combination of the CO2 with a 5-carbon
+ phosphate sugar. The rate at which this combination occurs depends on the
activity of the enzyme Rubisco. Assuming other nutrients such as phosphate not
limiting, Farquhar showed that this rate wc, proportional to the Rubisco, and two
other rates were potentially the slowest, hence controlling the overall process
rate. Because Rubisco is so slow, it is the most abundant protein in plants and
hence, presumably, the world’s largest supply of protein. The other two controlling
rates are those of photosynthate export wc and photon driven electron transport,
wj. The parameterization of we is also proportional to Rubisco and differs
primarily in temperature dependence such that it dominates at lower temperatures.
Hence, there are only two variables needed in addition to temperature and water
supply to model the leaf ET and carbon assimilation. These are the incidence of
solar energy on the leaf at visible wave lengths (PAR), and the concentration of
Rubisco.
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The incidence of PAR is modeled in climate models to varying degrees of
realism from the solar radiation passing through the atmosphere with the use of
canopy radiative transfer models. However, the Rubisco has previously been
provided to climate models implicitly as boundary condition, either by specifying
a minimum stomatal resistance parameter, r smin, or a maximum rate of
photosynthesis parameter, Vmax. The formation of Rubisco depends on leaf
uptake of nitrogen from the soil, and with no limitations on this uptake may be
optimized for plant requirements (e.g. Field, 1983). Dickinson et al. (2000) have
made a first attempt to include the Rubisco nitrogen in a climate model as a
prognostic variable. Although most previous treatments of leaf Rubisco dynamics
have simply assumed a proportionality to leaf nitrogen, fitting such an assumption
to observational data requires different proportionality constants for different
plant species. Data indicates that such a factor scales inversely with leaf specific
weight (i.e. leaf thickness), suggesting that much of the leaf nitrogen is required
to meet the needs of structure such as formation of cell walls.
The simplest assumption is that plants require a fixed nitrogen to carbon ratio
for their structural needs and all additional nitrogen goes to build proteins
proportional to Rubisco. With that, the Rubisco is established using only a plant
carbon model, and uptake of soil nitrogen. Any nitrogen uptake model must build
in feedbacks to recognize adequate leaf Rubisco. Otherwise, since much of the
leaf respiration can scale with Rubisco, the LAI and Rubisco would not equilibrate
until carbon loss by plant respiration matched carbon assimilation. An even more
serious code catastrophe can occur from respiration after a plant has lost most of
its leaves to cold or drought stress and again wants to start growing. Plants upon
losing leaves translocate about half of their nitrogen back to the plant and if this
can only go into Rubisco, respiration losses can easily kill any attempt to generate
new leaves. Hence, it was found that to be adequately realistic and robust in
interacting with a climate model, that leaf nitrogen has to be put into 3 separate
compartments, not only one for Rubisco but also for leaf structure and a labile
pool, the latter buffering Rubisco nitrogen from exceeding that put into leaf
structure.
Plants and the Rubisco pool cycle nitrogen with the soil pool on an annual
time scale. For some climate modeling purposes, total pool nitrogen could be
specified. However, on a decadal time scale, the soil nitrogen levels may undergo
large change by imbalance between sources and sinks. These sources and sinks
involve natural processes with feedbacks from climate model parameters and
from anthropogenic inputs. The later are primarily fertilization of agricultural
systems and atmospheric deposition of nitrate and ammonium nitrogen, coming
from industry (e.g. fossil fuel being originally from plants releases not only
carbon but oxides of nitrogen, and feedlots highly concentrate ammonium
compounds, also originally having been derived from plants). Planting of legumes
is also a major human contribution to global soil nitrogen. Because anthropogenic
inputs of nitrogen are nearly as large as natural sources, some authors (e.g.
Vitousek, 1993) have concluded that nitrogen may be as serious an environmental
perturbation as carbon dioxide. However, such a conclusion is presently
Linking Ground Hydrology to Ecosystems and Carbon Cycle in a Climate Model
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Plant Labile
Leaf
Structural
Rubisco
related
Uptake
Volitization
Denitrification
Root
Soil
Organic
Soil
Ammonium
Soil
Nitrate
Leaching
Fixation, Fertilization, Deposition
Fig. 2.
controversial. Our study did not attempt to address any such issues and put in the
anthropogenic sources at two constant rates, one for agricultural land use and one
for natural systems.
Figure 2 shows the plant and soil nitrogen reservoirs that are needed. Natural
generation of nitrogen by biological fixation depends on temperature and on the
supply of plant assimilated carbon to the soil. The later, and hence fixation, is
suppressed during drought stress. Nitrogen is lost primarily by leeching and
denitrification. Because these loses are almost entirely from nitrate ions, their
modeling requires distinction between soil ammonium and soil nitrate ions. The
ammonium ions are supplied from “mineralization” of an organic pool, as the
excess not needed for supplying the micro-biota that feed on the soil carbon. This
pool in turn is “nitrified” by appropriate microorganisms that feed on its hydrogen
to the nitrate pool.
Although many of the ingredients of the nitrogen cycling are commonly
treated in research literature on soil fertility and soil ecology, we could not locate
any good physical parameterizations for soil nutrient uptake suitable for coupling
to a climate model. Hence, developing the needed parameterization is one of the
more innovative aspects of our paper. Such a parameterization must include three
(3) rate processes. These are the active ion uptake at the root interface, and the
physical transports by diffusion and the ET driven bulk transport. The ammonium
ions are much less soluble than the nitrate and different plant species are
measured to have different ion uptake physiological parameters such as the
maximum rate per unit area, denoted Imax. Ion solubility was treated very crudely
as a reduction in the physical transport rates. We assumed single constants for all
the root uptake physiological parameters. There is probably not enough such data
on such to make the distinctions necessary for global vegetation.
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To test the parameterizations developed, they were incorporated into the
BATS land model and integrated with the NCAR CCM3 climate model, as forced
by 18 years of SSTS provided by the AMIPII project. All the soil biogeochemical
variables were treated as 1-box models; that is vertical details within the soil were
neglected. These included leaf, root, and soil carbon pools, and Rubisco, leaf
structure, and root, and various soil nitrogen pools. Initialization was first in
terms of fixed nitrogen to carbon ratios and was improved (spunup) through a
large number of repeats of the eighteen (18) year integration as we introduced
improvements piecemeal. We did not expect any highly visible changes in the
model climate and did not see any. Analysis primarily addressed the functioning
of the surface processes, and to some extent, validation against global land data.
It was interesting to see that the ET, nitrogen, and plant growth were highly
correlated. Such a correlation does not, by itself, establish strong controls of
nitrogen on the surface climate, but is suggestive of such.
Our conclusions were that the model ET was successfully coupled to
nitrogen, plant carbon fluxes and stores were reasonable, high latitudes showed
time scales too long to equilibrate over the model run, and that substantial
interannual variability of nitrogen cycling was obtained coupled to climate
variability.
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